Generator differential protection 1

[arozman]

We have following single pole schematics: 10.5 kV, 88MVA, 22% generator (grounded via resistors) is connected to step-up transformer (90MVA, 117/10.5 kV, YNd5, 11%) via bus bar connection. Between generator and transformer the generator circuit breaker and two feeders (for static excitation system and auxilliary power supply, 1MVA each) are located; excitation feeder is on generator side of circuit breaker, auxilliary power supply feeder is on transformer side of breaker. Transformer is connected to OHL via another (110 kV) breaker.

We shall utilize own differential protections for both machines: 87G for generator and 87 T for transformer.

Is it suitable to add the third differential protection, which should cover area between generator neutral and low voltage side of step-up transformer (including excitation feedder, generator circuit breaker, auxilliary power supply feeder)?

THANK YOU!!!

 

[davidbeach]

It would look like your center differential should be a three terminal differential rather than a two terminal, but why include the generator in that zone since you already have the generator covered?

 

[jbartos]

Suggestion: The differential protection is usually applied by IEEE Std 242 Buff Book, and other ANSI/IEEE Stds.
The differential protection is applicable to two or more conductor circuit, e.g. transformer, generator, transmission line, bus, etc.
If the intended differential protection is supposed to protect the generator and its output buses downstream to the transformer low voltage side, then there will be an overlap in differential protection between the generator differential protection and the intended differential protection, which is unnecessary. It is better to extent the existing generator differential protection 87G or existing transformer differential protection 87T to cover the differentially unprotected line between the generator and transformer.